Acetylation and deacetylation of histones in the chromatin is a key step in regulating the gene expression, whereas abnormal gene expression constitutes the molecular biological basis for the occurrence of tumors and some hereditary and metabolic diseases. The acetylation degree of histones is co-regulated by histone acetylases (HATs) and histone deacetylases (HDACs). Overexpression of HDACs and the recruitment thereof by transcription factors would cause abnormal inhibition of specific genes, leading to tumors and other diseases (Grunstein, M., 1997, Nature, 389:349-352). The HDAC activity is reported to be related to the occurrence of cancers, immunological diseases, and some mental and cardiovascular diseases (Ruijter, A-J-M., 2003, Biochem. J., 370:737-749; Grignani, F., 1998, Nature, 391:815-818; Lin, R-J, 1998, 391:811-814; Marks, P-A., 2001, Nature Reviews Cancer, 1:194).
It is experimentally demonstrated that the inhibitor of HDACs can increase the level of acetylation of histones in the chromatin, accordingly causing the activation and expression of specific genes and in turn the terminal differentiation of cells or the apoptosis of cancer cells. Preliminary clinical research has shown that it is safe for a human to achieve a high acetylation level of histones by inhibiting the HDAC activity. Therefore, HDACs are of the newest and hottest targets in the current research and development area for chemotherapeutic drugs.
HDAC is an enzyme superfamily with its members known to occur in four classes comprising 18 different subtypes, wherein Class I comprises four subtypes, namely HDAC1, 2, 3, and 8; Class II comprises six subtypes, namely HDAC4, 5, 6, 7, 9, and 10 (wherein HDAC4, 5, 7, and 9 belong to Class IIa, and HDAC6 and 10 belong to Class IIb); Class IV only comprises one subtype, HDAC11 which shares certain homology with the previous two classes; Class III comprises seven subtypes in total including Sirt1-7 which do not share any structural homology with the previous three classes. At present, HDAC subtypes in Class I and Class II are relatively better studied, but not much is known about Class III and Class IV of HDACs. HDAC Class I proteins mainly localize in the nucleus and are expressed in cells of multiple tissues. HDAC Class II proteins mainly localize in the cytoplasm (or shuttle between the nucleus and the cytoplasm) and are only expressed in cells of some kinds of tissues. Results from research using small interfering RNA technique and animal gene knockout technique have shown comprehensively that some HDAC subtypes from Class I and Class IIa could be the target relevant to an antitumor effect, wherein inhibiting HDAC1, 2 and 3 can result in inhibition of cell proliferation; inhibiting HDAC4 can affect the repair processes of DNA damage; and inhibiting HDAC7 can induce apoptosis of thymocytes.
Research in recent years has fully demonstrated that the overexpression or abnormal activities of HDACs play an important role in the occurrence and development of leukemia and solid tumors. It is shown that significant in vivo and in vitro antitumor effects can be achieved by inhibiting the HDAC functional activity. HDAC inhibitors currently under investigation in clinical trials can be categorized into four groups based on their chemical structures, namely:
(1) hydroxamic acids, e.g. trichostatin (TSA), and suberolanilide hydroxamic acid (SAHA);
(2) cyclic tetrapeptides, e.g. Apicidin;
(3) short-chain or aromatic fatty acids, e.g. sodium butyrate; and
(4) benzamides, e.g. MS-275.
The first two groups of compounds are non-selective HDAC inhibitors and inhibit all HDAC subtypes in Class I and Class II. Short-chain or aromatic fatty acids generally have weaker inhibition activities and need to be tested in combination with other medicaments to provide stronger effects. Benzamides, however, are selective with targeting effects and mainly inhibit Class I HDACs (including HDAC subtypes 1, 2 and 3 but not HDAC8) and part of Class IIa HDACs and have no inhibitory effect on Class IIb HDACs. At present, antitumor drugs targeting HDACs are being investigated worldwide. SAHA (Trade name Zolinza) developed by Merk Company has been approved by FDA for treating cutaneous T-cell lymphoma (CTCL) and launched at the end of 2006. This not only indicates the end of the confirmatory research phase for the concept of using HDAC as a novel drug target, but also implies a broad prospect of developing HDAC inhibitors as novel antitumor drugs. Meanwhile, studies on the mechanisms underlying the antitumor effect of HDAC inhibitors are gradually going deeper and encompass multiple aspects related to the tumor formation and development such as induction of tumor cell apoptosis, inhibition of tumor cell cycle, induction of tumor cell differentiation, inhibition of angiogenesis, inhibition of tumor metastasis, and regulation of the immune system function etc.
Due to the structure similarity of the HDAC subtypes, most of the present HDAC inhibitors are not selective towards these subtypes and typically inhibit multiple subtypes at the same time, causing certain side effects and thus affecting the druggability of the inhibitors. Consequently, research focus and technical difficulty in the prior art of this area is to obtain a novel and effective anti-malignant tumor agent with low adverse side effects and high safety by designing and synthesizing highly selective HDAC inhibitors, especially the HDAC inhibitors of benzamides.
It is shown by the study that the structures of most of the HDAC inhibitors comprise an enzyme surface recognition domain, a linker region and a zinc-chelating region. Studies on the zinc-chelating region of the N-benzanilides indicate that 2′-amino group is the key group for the inhibitory activity, because the compound would lose its inhibitory activity when the 2′-amino group is removed and the 3′- or 4′-position on the aryl is substituted with an amino or acetylated amino group, indicating that an amino group at the ortho position is necessary for the compounds' inhibitory activities. The length, hydrophobicity and steric hindrance of the groups in the linker region and the enzyme surface recognition domain can also affect the inhibitory activity of the compound toward the enzyme. Studies on the linker region and the enzyme surface recognition domain have revealed that: the planar or SP2 configuration of the linking unit is essential for effective HDAC inhibition, so the aromatic ring (benzene ring) of rigid planar structure in the linker region should be kept; and the binding between the enzyme surface recognition domain and the target enzyme is achieved mainly through hydrogen bonding between the substituents and the amino acid residues, so the inhibitory activity of the compound can be enhanced by increasing the hydrophobicity of this domain. A series of novel building blocks can be obtained through diverse modifications of the substituents on the aromatic ring or the aromatic heterocyclic ring.